Aqueous drilling fluids containing fluid loss additives

ABSTRACT

Copolymers of (1) from about 80% to about 98% by weight of acrylic acid and (2) from about 2% to about 20% by weight of itaconic acid and having a weight average molecular weight of between about 50,000 to about 1,000,000, preferably between about 100,000 to about 500,000, and which are at least water dispersible, are used as fluid loss control additives for aqueous drilling fluids and are particularly advantageous when used with muds containing soluble calcium and muds containing chloride ions such as seawater muds. An example is a copolymer of 95% by weight acrylic acid and 5% by weight itaconic acid in its sodium salt form.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to particular copolymers of acrylic acid and itaconic acid which are at least water dispersible and their use as fluid loss control additives for aqueous drilling fluids which are used in drilling wells in subterranean formations.

2. Description of the Prior Art

Rotary systems are generally used in drilling oil and gas wells. These systems depend upon rotation of a string of drill pipe to the bottom of which is attached a drill bit. The bit cuts into the formation causing the cuttings to accumulate as drilling continues. A drilling fluid is used which lubricates the drill bit and carries cuttings to the surface for removal. This allows the bit to continue functioning and the bottom hole to be clean and free of cuttings. The drilling fluid is also the source of pressure which is exerted against the formation. Even in other drilling systems, drilling fluids are still needed to remove bore hole cuttings and to otherwise perform functions related to drilling fluids.

Oil and gas producing formations are generally porous layers having varying degrees of permeability to the flow of fluids such as oil, water or gas. When drilling through such porous formations, it is essential to employ drilling fluids having characteristics such that excessive amounts of the drilling fluids do not penetrate the porous formation. Drilling fluids have a tendency to penetrate the formation because pressure in the bore hole is greater than the pressure opposing it which comes from the formation. Should excess penetration occur, there is loss of drilling fluid into the formation resulting in loss of pressure, inability to drill and remove cuttings and in the case of loss from brine muds, extra expense because of their cost. Mere filter cake formation on the wall of the bore hole does not adequately eliminate fluid loss.

According to U.S. Pat. No. 4,293,427, Lucas et al, Oct. 6, 1981, various materials have been added to drilling fluids to control fluid loss and have included materials such as pregelatinized starch, sodium carboxymethyl cellulose, sodium polyacrylates and lignites.

U.S. Pat. No. 2,718,497--Oldham et al, Sept. 20, 1955, discloses that polymers of acrylic acid when added to drilling muds reduce fluid loss of same. The muds to which the polymer is added do not contain calcium or chloride ions.

C. S. Stanley in World Oil, July 1959 (pages 122-128) describes studies, including fluid loss studies, of acrylic acid-acrylamide copolymers of various carboxylate/amide ratios and molecular weights on mud formulations.

C. S. Stanley in American Chemical Society, Division Petroleum Chemical Preprints 7(1), pages 65-70 (1962), describes fluid loss control studies on clay-water suspensions used as oil well drilling fluids using a copolymer of about 200,000 molecular weight consisting of about 70 mole % sodium acrylate and 30 mole % acrylamide.

CA 87:25579e (1977) discloses a water loss reducer and stabilizer of drilling fluids prepared by partial hydrolysis of an acrylonitrile itaconic acid copolymer.

CA 52:20981h (1958) discloses salts of polymethacrylic or polyacrylic acid used as modifiers for cement or mortar, including salts of copolymers of methacrylic or acrylic acid and maleic, fumaric or itaconic acid.

CA 93:120386x (1980) discloses synthesis of polycarboxylic acids and the properties of cement formed by their reaction with ion-leachable glasses. A copolymer of acrylic and itaconic acids combined several favorable characteristics.

CA 99:110801p (1938) discloses dental cements containing glass particles coated with polymers. Acrylic acid-itaconic acid copolymer is disclosed.

Where polyacrylates are added to fresh water muds, fluid loss can be reasonably controlled. However, where soluble calcium or chloride ions are present in the muds, generally in excess of about 300 ppm of soluble calcium ions and in excess of about 1,000 ppm of chloride ions, as for example from the cement or from the formation in the case of the former and from seawater (brine) and salt water in the case of the latter, particularly under conditions of high temperature and pressure, polyacrylates are not as effective in controlling fluid loss.

SUMMARY OF THE INVENTION

Particular copolymers of acrylic acid and itaconic acid which are at least water dispersible have been found to be outstanding fluid loss control additives, not only for fresh water muds, but also for muds having high soluble calcium and chloride concentrations, especially under conditions of high temperature and pressure.

These copolymers are added to water based drilling fluids, usually muds, which are then used in the conventional manner. Drilling fluids, particularly muds containing soluble calcium ions in excess of about 300 ppm and muds containing chloride ions in excess of about 1,000 ppm, which contain these copolymers are characterized by outstanding fluid loss properties when employed under conditions of high temperature and pressure such as in excess of 300° F. and 500 psi. Thus, these copolymers are not only as effective as polyacrylate homopolymers when used as fluid loss additives with fresh water muds, but are far superior than polyacrylate homopolymers when used as fluid loss additives for high calcium and high chloride muds under conditions of high temperature and pressure.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The Copolymers

The copolymers are copolymers of (1) from about 80% to about 98% by weight of acrylic acid and (2) from about 2% to about 20% by weight of itaconic acid and are at least water dispersible, i.e., water dispersible or water soluble. They have a weight average molecular weight of between about 50,000 to about 1,000,000, preferably between about 100,000 to about 500,000. They can be used in the free acid form or in the form of any partially or completely neutralized salt or quaternary salt form so long as they are at least water dispersible. Examples are the sodium, potassium and ammonium salts. Of course, the form of the copolymer is dependent upon the pH of the mud. For example, if the copolymer is introduced into the mud in its free acid form, as a result of the alkalinity of the mud, it may actually be present in the mud in the form of its partially or completely neutralized salt form.

These copolymers are present in the various drilling fluids including soluble calcium and chloride ion containing muds in amount sufficient to reduce fluid loss of the drilling fluid, for example, in amounts of from about 0.5 to about 10 pounds, preferably from about 2 to about 6 pounds per 42 gallon barrel of drilling fluid.

The copolymers can be prepared from their respective monomers by conventional polymerization procedures, viz., solution, emulsion or bulk polymerization in the presence of conventional free radical initiators. Such procedures are well documented in the literature and do not form a part of this invention.

Examples of copolymers are (percents are weight percents):

sodium salt of the copolymer of acrylic acid (95%) and itaconic acid (5%)

potassium salt of the copolymer of acrylic acid (90%) and itaconic acid (10%)

free acid of the copolymer of acrylic acid (80%) and itaconic acid (20%)

ammonium salt of the copolymer of acrylic acid (90%) and itaconic acid (10%)

Optionally, the dry copolymer in free acid or partially neutralized form can be admixed with carbonate, e.g., sodium carbonate (soda ash), in amounts up to about 40% by weight in order to bring about complete neutralization of the copolymer and eliminate subsequent pH adjustment of the mud containing same. The copolymer can be added to the drilling fluid formulations in dry, solution or emulsion form.

Drilling Fluids

The copolymers can be used with a wide variety of water based drilling fluids such as clear drilling fluids, low solids muds, bentonite and weighted muds. Muds can be classified as fresh water, soluble calcium ion and chloride ion containing muds. The latter two generally have present soluble calcium ions in excess of about 300 ppm and chloride ions in excess of about 1,000 ppm, respectively. These drilling fluids, including the muds and their preparation, are well known in the drilling art. For purposes of evaluation, the following procedures were used to prepare muds having the compositions set forth in Table I.

Fresh water mud was prepared by slowly adding a clayey material, bentonite, with high shear mixing, e.g., using a Premier Series 2000 Dispersator from Premier Mill Corp., to distilled water followed by mixing for one hour. Thereafter, the resulting bentonite slurry was allowed to hydrate for 48 to 72 hours.

Calcium mud and seawater mud I were prepared by adding the required amount of calcium chloride or sodium chloride plus sea salt to bentonite as prepared above followed by stirring for 30 to 45 minutes.

Thinner base mud was prepared by adding sodium chloride and calcium carbonate to distilled water while mixing. Thereafter, the following clay materials were added slowly in the following order: rev dust, Wyoming bentonite and calcium bentonite. Each clay was then mixed for 10 minutes at medium speed with a Premier Series 2000 Dispersator before adding the next clay. After adding the last clay, the mix was stirred for 30 minutes at medium speed and then aged overnight at room temperature.

Seawater mud II was prepared by adding sea salt to thinner base mud and then stirring the mixture for 45 minutes.

In the case of all muds, before obtaining a sample from stock for polymer evaluation, the mud is stirred for 2 to 3 minutes to ensure homogeneity and the pH of the mud is adjusted individually to pH 10 before adding the polymer.

The mud compositions appear below.

                                      TABLE I                                      __________________________________________________________________________     MUD COMPOSITIONS                                                                                         Thinner        2000 PPM                                      Fresh Water Mud                                                                         Seawater Mud I                                                                          Base Mud                                                                             Seawater Mud II                                                                         Ca.sup.++  Mud                        Ingredients                                                                            (% by wt.)                                                                              (% by wt.)                                                                              (% by wt.)                                                                           (% by wt.)                                                                              (% by wt.)                            __________________________________________________________________________     Water   93.96         78.81                                                                              85.65      82.77                                                                              93.27                                 Wyoming  6.04          2.29                                                                              6.12        5.91                                                                               6.00                                 bentonite                                                                      OCMA clay*                                                                             --            16.02                                                                              --         --  --                                    NaCl             1.60     0.21  0.21                                           Salt    --             2.88           3.59                                                                              --                                    Sea salt         1.28     --    3.38                                           Calcium --            --  5.14        4.96                                                                              --                                    bentonite                                                                      REV dust**                                                                             --            --  2.45        2.36                                                                              --                                    Calcium --            --  0.43        0.41                                     carbonate                                                                      CaCl.sub.2.2H.sub.2 O                                                                  --            --  --         --   0.73                                 Total   100.00        100.00                                                                             100.00     100.00                                                                             100.00                                pH      10.00         10.40                                                                              10.00      10.00                                                                              10.00                                 __________________________________________________________________________      *Mixture of kaolin and china clay                                              **Drilling solids which is a mixture of barite and clay                  

Evaluation of Copolymers as Fluid Loss Additives

The following procedures were utilized to evaluate the copolymers as fluid loss additives in the various drilling muds.

Preparation of Test Samples (Polymer Plus Mud)

350 ml of mud is placed in a stainless steel beaker. The required amount of polymer is weighed in a weighing dish (1 gram of polymer in 350 ml mud equals 1 pound per barrel). With thorough mixing, the polymer is added slowly to the mud and mixed for an additional 15-20 minutes. The pH of the mud is monitored during mixing and adjusted to pH 10, if necessary. After the mixing period, viscosities and gel strength of the mud can be measured if desired using a Fann viscometer (Model 35-A). Fluid loss measurements are then run on the samples.

API Fluid Loss Test

Equipment: IMCO Standard Filter Press #R1100 (supplied by Imco Services Div. of Halliburton Company). Nitrogen gas cylinder for pressurizing the fluid loss cell.

Test Conditions: 77° F., 100 psi pressure. Collect filtrate for 30 minutes.

Procedure:

1. The mud sample is placed in the cell of the filter press, cell filled to within 1/4" of the top.

2. The cell is placed in the filter press frame.

3. 100 psi pressure (using nitrogen gas) is applied to the filter cell and timing of the test is started.

4. At the end of 30 minutes, the volume of filtrate collected is read. The lower the volume of filtrate collected, the more effective is the polymer in preventing loss of fluid from the mud formulation.

High Temperature High Pressure Fluid Loss Test

This test method is described in Baroid Drilling Mud Data Book, Section 900, pp. 26-27 entitled, "High Temperature Filtration Testing for Drilling Fluids." The following summarizes the equipment, conditions and procedure.

Equipment: HPHT Filter Press, Model #387 (supplied by NL Baroid, Houston, Tex.), which includes cell and heating jacket.

Test Conditions: Temperature can be varied from 200°-400° F. Pressure is normally set at 500 psi differential (600 psi at top, 100 psi back pressure at bottom).

Procedure:

(a) The mud sample is placed into the fluid loss cell.

(b) The cell is placed into the heating jacket.

(c) Top and bottom pressure lines are connected.

(d) After ensuring that the stems on the fluid loss cell are shut, 250 psi top and 100 psi bottom pressure is applied using nitrogen gas.

(e) The cell is heated to the required temperature (in about one hour's time) and equilibrated at the temperature for an additional half hour.

(f) The bottom stem is opened, then the top stem and any collected fluid is bled out.

(g) Top pressure is set to 600 psi.

(h) Time is recorded and fluid loss readings are taken at intervals of 1, 3, 71/2 and 30 minutes.

(i) Total volume of filtrate collected after 30 minutes is recorded and multiplied by two (correction for 3.5 sq. inch filtration area versus 7 sq. inch area).

Heat Aging Test

Equipment: Aging cell, Model #76017 obtained from NL Baroid, Houston, Tex. Roller or forced air oven.

Procedure:

(a) Mud containing polymer is placed in aging cell.

(b) The contents of the cell are pressurized with nitrogen gas using the recommended pressures provided in FIG. 29, page 900-32 of "Mud Volume and Pressure for High Temperature Aging", NL Baroid, Drilling Mud Data Book, Section 900.

(c) The cell is then placed in a roller oven pre-heated to the desired heat aging temperature (dynamic heat aging) or a forced air oven (static heat aging) for 16 hours. All testing is dynamic heat aging unless indicated otherwise.

(d) At the end of the heat aging period, the aging cell is removed from the oven, cooled under cold running water, pressure released and the aged mud transferred into a stainless steel beaker.

(e) The pH of the mud is checked and if necessary adjusted to between 8.5 to 10. Rheology measurements are run using Fann viscometer, Model 35-A followed by the fluid loss test which is either the API or high temperature high pressure fluid loss test described previously.

Mud Rheology

Plastic viscosity (PV), yield point (VP) and gel strengths at 10 second and 10 minute intervals were obtained as described in Applied Mud Technology, 7th Edition (1981), pages 3 and 4, published by Imco Services.

Molecular Weights

Molecular weights were determined by gel permeation chromatography using polystyrene sulfonic acid as the standard.

For a fuller understanding of the nature and advantages of this invention, reference may be made to the following examples. These examples are given merely to illustrate the invention and are not to be constructed in a limiting sense.

In the examples all references to pounds per barrel of drilling fluid are to a 42 gallon barrel of the drilling fluid.

EXAMPLE I Homopolymer of Acrylic Acid

For purposes of comparison with the copolymers of this invention, an additive, viz., a homopolymer of acrylic acid having a weight average molecular weight of about 250,000-300,000, was evaluated as a fluid loss control additive in a mud containing 500 ppm calcium ions. Concentration was 2 pounds per barrel. The additive was a mixture of polyacrylic acid (70% by wt.) and soda ash (30% by wt.). Thus, the polymer was partially neutralized. Fluid loss evaluation was conducted at 300° F., 500 psi for 30 minutes without heat aging. Fluid loss was in excess of 120 ml indicating that the homopolymer of acrylic acid is not very effective under these conditions.

EXAMPLE II Preparation of Acrylic Acid--Itaconic Acid Copolymer (95/5 Weight %)

61.5 parts by weight of glacial acrylic acid was mixed with 10.5 parts by weight of 31% by weight sodium itaconate solution. The solution was partially neutralized with 3 parts by weight soda ash and then polymerized adiabatically using ammonium persulfate (0.45 parts by weight)/sodium metabisulfite (0.45 parts by weight)/sodium thiosulfate (0.8 parts by weight) as redox catalyst. At the end of the polymerization, the product was dried and ground (≦20 mesh) for evaluation. The copolymer was a partially neutralized sodium salt.

The weight average molecular weight of this copolymer is about 280,000.

The copolymers in Table II below, in their partially neutralized sodium salt form, were prepared in the same manner as the copolymer of Example II above. Variations in reactants are tabulated below. All quantities are parts by weight (pbw).

                                      TABLE II                                     __________________________________________________________________________               31% pbw Soln                                                                               Ammonium                                                                             Sodium                                                                              Sodium                                                                             Wt. %                                          Acrylic                                                                             Sodium  Soda                                                                               Per-  Meta-                                                                               Thio-                                                                              Acrylic Acid/                                  Acid Itaconate                                                                              Ash sulfate                                                                              bisulfite                                                                           sulfate                                                                            Itaconic                                  Ex. No.                                                                             (pbw)                                                                               (pbw)   (pbw)                                                                              (pbw) (pbw)                                                                               (pbw)                                                                              Acid                                      __________________________________________________________________________     II A*                                                                               61.5 10.5    3   0.22  0.22 0.4 95/5                                      II B**                                                                              61.5 10.5    3   0.15  0.15 0.3 95/5                                      II C 61.5 22.6    3   0.45  0.45 0.8  90/10                                    II D 78.0 6.8     4   0.55  0.55 0.63                                                                               97.5/2.5                                  __________________________________________________________________________      *Wt. average molecular weight is 370,000                                       **Wt. average molecular weight is 400,000                                

In the following tables, fluid loss data is presented for muds containing copolymers of acrylic acid-itaconic acid in partially or completely neutralized sodium salt form as shown.

For purposes of comparison, the blanks (no fluid loss control additive present) for high calcium, high chloride muds were exposed to high temperature, high pressure fluid loss conditions for thirty minutes. Fluid loss was ≧200 ml as shown in Table III below.

                                      TABLE III                                    __________________________________________________________________________     (No fluid loss control additive present)                                                Fluid Loss Test Conditions Mud Rheology                                        16 Hr Heat    Fluid Loss         10 Sec                                                                             10 Min                                    Aging Temp                                                                            Temp                                                                               Pr After 30 Min                                                                          Filter                                                                               PV YP Gel Gel                              Mud      (°F.)                                                                          (°F.)                                                                       (psi)                                                                             (ml)   Cake  (cps)                                                                             (lbs/100 ft.sup.2)                      __________________________________________________________________________     Fresh water                                                                             --      77 100                                                                               13.5   --    33 215                                                                               144 145                              Fresh water                                                                             150     77 100                                                                               13.4   --    79 62 59  84                               Seawater II                                                                             --     200 500                                                                               190    9/16" cake                                                                           7  25 17  17                               2,000 ppm calcium                                                                       --     200 500                                                                               190    8/16" cake                                                                           6  46 16  16                               2,000 ppm calcium                                                                        350*  200 500                                                                               230          7  22 12  12                               __________________________________________________________________________      *Static heat aged                                                        

                                      TABLE IV                                     __________________________________________________________________________                             Fluid Loss Test Conditions                             Copolymer of Ex. II     16 Hr       Fluid                                      Monomer                                                                              Component         Heat        Loss        Mud Rheology                   Acrylic                                                                              Itaconic          Aging       After              10                                                                                 10 Min              Acid  Acid  Dosage      Temp                                                                               Temp                                                                               Pr  30 Min                                                                             Filter  PV  YP Gel Gel                 (wt %)                                                                               (wt %)                                                                               lbs/bbl                                                                             Mud    (°F.)                                                                       (°F.)                                                                       (psi)                                                                              (ml)                                                                               Cake    (cps)                                                                              (lbs/100                   __________________________________________________________________________                                                         ft.sup.2)                  95    5     5*   Fresh water                                                                           150 77  100 5.5 3/32" soft, wet                                                                        72  68 14  16                  95    5     5*   Fresh water                                                                           300 77  100 6.2 3/22" soft, wet                                                                        54  58 13  13                  95    5     5*   Fresh water                                                                           400 77  100 7.4 2/32" soft, wet                                                                        44  42 10  10                  __________________________________________________________________________      *Copolymer present as 70/30 wt. % blend of copolymer/soda ash which            results in a 70% neutralized copolymer (sodium salt form).               

                                      TABLE V                                      __________________________________________________________________________                             Fluid Loss Test Conditions                             Copolymer of Ex. II     16 Hr       Fluid                                      Monomer                                                                              Component         Heat        Loss        Mud Rheology                   Acrylic                                                                              Itaconic          Aging       After              10                                                                                 10 Min              Acid  Acid  Dosage*     Temp Temp                                                                               Pr 30 min                                                                             Filter  PV  YP Gel Gel                 (wt %)                                                                               (wt %)                                                                               lbs/bbl                                                                             Mud    (°F.)                                                                        (°F.)                                                                       (psi)                                                                             (ml)                                                                               Cake    (cps)                                                                              (lbs/100                   __________________________________________________________________________                                                         ft.sup.2)                  95    5     3    2000 ppm Ca                                                                           150   77 100                                                                               67  16/32" uniform                                                                          9   8 7   11                  95    5     3    2000 ppm Ca                                                                           400   77 100                                                                               59  18/32" uniform                                                                          9   6 6   34                  95    5     3    2000 ppm Ca                                                                           400  300 500                                                                               56  12/16" uniform                                                                         21  10 2    8                  95    5     5    2000 ppm Ca                                                                           --   200 500                                                                               16   4/32" uniform                                                                         45  32 6   16                  95    5     5    2000 ppm Ca                                                                            350**                                                                              200 500                                                                               16   2/32" uniform                                                                         28  13 5    6                  __________________________________________________________________________      *Copolymer present as 70/30 wt. % blend of copolymer/soda ash which            results in a 70% neutralized copolymer (sodium salt form).                     **Static heat aged                                                       

    Fluid Loss Test Conditions                                                     Copolymer of Ex. II*    16 Hr       Fluid                                      Monomer                                                                              Component         Heat        Loss        Mud Rheology                   Acrylic                                                                              Itaconic          Aging       After              10                                                                                 10 Min              Acid  Acid  Dosage      Temp Temp                                                                               Pr 30 Min                                                                             Filter  PV  YP Gel Gel                 (wt %)                                                                               (wt %)                                                                               lbs/bbl                                                                             Mud    (°F.)                                                                        (°F.)                                                                       (psi)                                                                             (ml)                                                                               Cake    (cps)                                                                              (lbs/100                   __________________________________________________________________________                                                         ft.sup.2)                  95    5     4    2000 ppm Ca                                                                           --   300 500                                                                               112 Gelled  43  26 10  24                  95    5     4    2000 ppm Ca                                                                           400  300 500                                                                               59  Gelled  31  19 11  79                  95    5     5    2000 ppm Ca                                                                           --   300 500                                                                               24   4/32" uniform                                                                         36  15 4   11                  95    5     5    2000 ppm Ca                                                                           400  300 500                                                                               28   6/32" uniform                                                                         19   5 3    4                  95    5     6    2000 ppm Ca                                                                           --   300 500                                                                               22   2/32" uniform                                                                         54  26 20  44                  95    5     6    2000 ppm Ca                                                                           400  300 500                                                                               20  10/32" uniform                                                                         38  44 43  68                  __________________________________________________________________________      *Partially neutralized copolymer (sodium salt)                           

                                      TABLE VI                                     __________________________________________________________________________     Seawater Mud II                                                                Copolymer of Ex. II                                                                              Fluid Loss Test Conditions    Mud Rheology                   Monomer                                                                               Component  16 Hr Heat     Fluid Loss            10                                                                                 10 Min              Acrylic Acid                                                                          Itaconic Acid                                                                         Dosage                                                                             Aging Temp                                                                            Temp                                                                               Pr  After 30 min                                                                          Filter  PV  YP Gel Gel                 (wt %) (wt %) lbs/bbl                                                                            (°F.)                                                                          (°F.)                                                                       (psi)                                                                              (ml)   Cake    (cps)                                                                              (lbs/100                   __________________________________________________________________________                                                         ft.sup.2)                  95     5      5*  --     300 500 42      7/16" uniform                                                                         10   1 1    1                  95     5      5*  400    300 500 31      6/16" uniform                                                                          9  11 9   16                  95     5      6*  --     300 500 33      5/16" uniform                                                                         34  60 20  42                  95     5      6*  400    300 500 32      6/16" soft                                                                             9  11 8   13                  95     5       5**                                                                               --     200 500 21      2/32" uniform                                                                         23  58 21  43                  95     5       5**                                                                                 350***                                                                              200 500 22      2/32" uniform                                                                         12  10 5   10                  95     5       3**                                                                               150     77 100 50     14/32" uniform                                                                         15   8 11  25                  95     5       3**                                                                               300     77 100 49     14/32" uniform                                                                         10   6 8   27                  95     5       3**                                                                               400     77 100 35      6/32" uniform                                                                          8   9 12  32                  __________________________________________________________________________      *Copolymer present as partially neutralized copolymer (sodium salt)            **Copolymer present as 70/30 wt. % blend of copolymer/soda ash which           results in a 70% neutralized copolymer (sodium salt form).                     ***Static heat aged                                                      

                                      TABLE VII                                    __________________________________________________________________________     Seawater Mud I                                                                 Copolymer           Fluid Loss Test Conditions   Mud Rheology                  Monomer                                                                               Component    16 Hr Heat    Fluid Loss           10                                                                                 10 Min              Acrylic Acid                                                                          Itaconic Acid                                                                          Dosage*                                                                             Aging Temp                                                                            Temp                                                                               Pr After 30 min                                                                          Filter  PV YP Gel Gel                 (wt %) (wt %)  lbs/bbl                                                                             (°F.)                                                                          (°F.)                                                                       (psi)                                                                             (ml)   Cake    (cps)                                                                             (lbs/100                   __________________________________________________________________________                                                         ft.sup.2)                  95     5 Copolymer                                                                            2    --     300 500                                                                               40     5/16" uniform                                                                          34 24 16  43                  95     5 of Ex II B                                                                           2    400    300 500                                                                               64     6/16" uniform                                                                          14 29 22  27                  95     5 Copolymer                                                                            2    --     300 500                                                                               46     5/16" uniform                                                                          23 25 14  30                  95     5 of Ex II A                                                                           2    400    300 500                                                                               68     5/16" uniform                                                                          14 27 13  33                  95     5 Copolymer                                                                            2    --     300 500                                                                               46     4/16"  uniform                                                                         22 17 14  30                  95     5 of Ex II                                                                             2    400    300 500                                                                               86     5/16" uniform                                                                          12 35 29  35                  __________________________________________________________________________      *Copolymer present as 70/30 wt. % blend of copolymer/soda ash which            results in a 70% neutralized copolymer (sodium salt form).               

                                      TABLE VIII                                   __________________________________________________________________________     Seawater Mud I                                                                 Copolymer            Fluid Loss Test Conditions Mud Rheology                   Monomer                                                                               Component     16 Hr Heat    Fluid Loss          10                                                                                 10 Min              Acrylic Acid                                                                          Itaconic Acid                                                                           Dosage*                                                                             Aging Temp                                                                            Temp                                                                               Pr After 30 min                                                                          Filter                                                                               PV YP  Gel Gel                 (wt %) (wt %)   lbs/bbl                                                                             (°F.)                                                                          (°F.)                                                                       (psi)                                                                             (ml)   Cake  (cps)                                                                             (lbs/100                    __________________________________________________________________________                                                        ft.sup.2)                   95      5 Copolymer                                                                            2    --     300 500                                                                               48     Wet, loose                                                                           34 11  12  21                  95      5 of Ex II                                                                             2    300    300 500                                                                               57                                          95      5       2    400    300 500                                                                               58     Wet, loose                                                                           19 12   8  15                  90     10       2    --     300 500                                                                               62     --    32 22  13  32                  90     10 Copolymer                                                                            2    300    300 500                                                                               77     --    19 22  15  32                  90     10 of Ex II C                                                                           2    400    300 500                                                                               108    --    -- --  --  --                  __________________________________________________________________________      *Copolymer present as 70/30 wt. % blend of copolymer/soda ash which            results in a 70% neutralized copolymer (sodium salt form).               

                                      TABLE IX                                     __________________________________________________________________________     Seawater Mud I                                                                 Copolymer of Ex. II                                                                               Fluid Loss Test Conditions    Mud Rheology                  Monomer                                                                               Component   16 Hr Heat    Fluid Loss            10                                                                                 10 Min              Acrylic Acid                                                                          Itaconic Acid                                                                         Dosage*                                                                             Aging Temp                                                                            Temp                                                                               Pr After 30 min                                                                          Filter   PV YP Gel Gel                 (wt %) (wt %) lbs/bbl                                                                             (°F.)                                                                          (°F.)                                                                       (psi)                                                                             (ml)   Cake     (cps)                                                                             (lbs/100                   __________________________________________________________________________                                                         ft.sup.2)                  95     5      2    --     300 500                                                                               54     Wet, loose                                                                              34 11 12  21                  95     5      2    400    300 500                                                                               58     Wet, loose                                                                              19 12  8  15                  95     5      3    --     300 500                                                                               38      8/32" wet, loose                                                                       23  9  4   8                  95     5      3    400    300 500                                                                               36      8/32" wet, loose                                                                       15 23 22  33                  95     5      4    --     300 500                                                                               23      8/32" wet                                                                              33 16  5   8                  95     5      4    400    300 500                                                                               42     12/32" wet                                                                              25 29 23  32                  95     5      5    --     300 500                                                                               20      7/32" wet, loose                                                                       37 16  5   8                  95     5      5    400    300 500                                                                               36     12/32" wet                                                                              26 27 19  33                  __________________________________________________________________________      *Copolymer present as 70/30 wt. % blend of copolymer/soda ash which            results in a 70% neutralized copolymer (sodium salt form).               

                                      TABLE X                                      __________________________________________________________________________     Copolymer of Ex. II D*                                                         Monomer                                                                              Component        Fluid Loss Test Conditions                                                                               Mud Rheology                  Acrylic                                                                              Itaconic         16 Hr Heat    Fluid Loss        10                                                                                 10 Min              Acid  Acid  Dosage     Aging Temp                                                                            Temp                                                                               Pr After 30 min                                                                          Filter                                                                              PV YP Gel Gel                 (wt %)                                                                               (wt %)                                                                               lbs/bbl                                                                            Mud    (°F.)                                                                          (°F.)                                                                       (psi)                                                                             (ml)   Cake (cps)                                                                             (lbs/100                   __________________________________________________________________________                                                         ft.sup.2)                  97.5  2.5   5   2000 ppm Ca                                                                           400    300 500                                                                               64 ml  3/8" thick                                                                          40 22  7  23                  --    --    --  2000 ppm Ca                                                                           400    300 500                                                                                196 ml**                                                                             --    7 48 17  17                  __________________________________________________________________________      *partially neutralized copolymer (sodium salt)                                 **51/2 minutes                                                                 Note:                                                                          This copolymer was found not to be very effective in thinner base seawate      mud.                                                                     

EXAMPLE III

The partially neutralized ammonium salt of 95 weight % acrylic acid and 5 weight % itaconic acid was prepared as described in Example II except that ammonium itaconate was used instead of sodium itaconate and ammonium carbonate was used instead of soda ash.

Table XI sets forth fluid loss data in seawater mud II for this polymer.

                                      TABLE XI                                     __________________________________________________________________________     Copolymer*              Fluid Loss Test Conditions                                                                              Mud Rheology                  Monomer                                                                               Component        16 Hr Heat    Fluid Loss       10                                                                                 10 Min              Acrylic Acid                                                                          Itaconic Acid                                                                         Dosage    Aging Temp                                                                            Temp                                                                               Pr After 30 min                                                                          Filter                                                                             PV YP Gel Gel                 (wt %) (wt %) lbs/bbl                                                                            Mud   (°F.)                                                                          (°F.)                                                                       (psi)                                                                             (ml)   Cake                                                                               (cps)                                                                             (lbs/100                   __________________________________________________________________________                                                         ft.sup.2)                  95     5      4   Seawater                                                                             --     300 500                                                                               82 ml  10/16"                                                                             9  3  1   7                                     II                                                           __________________________________________________________________________      *Partially neutralized copolymer (ammonium salt)                         

EXAMPLE IV

Additional comparisons have been made between a copolymer of Example II (95 weight % acrylic acid and 5 weight % of itaconic acid) and commercial sodium acrylate-acrylamide copolymer of approximately 70 mole or weight % sodium acrylate and 30 mole or weight % acrylamide having a molecular weight of about 200,000 with respect to their fluid loss properties in drilling fluids containing high calcium and chloride ions. The data indicates that the sodium acrylate-acrylamide copolymer is not as effective as the acrylic acid--itaconic acid copolymers under these more severe conditions. The data appears in Table X below.

The sodium acrylate-acrylamide copolymer for convenience is referred to as AA-AM in the Tables while the acrylic acid-itaconic acid copolymer for convenience is referred to as AA-ITA in the Tables and was a partially neutralized sodium salt.

                                      TABLE XII                                    __________________________________________________________________________                       Fluid Loss Test Conditions    Mud Rheology                                     16 Hr Heat     Fluid Loss            10                                                                                 10 Min              Copolymer                                                                             Dosage     Aging Temp                                                                            Temp                                                                               Pr  After 30 min                                                                          Filter  PV  YP Gel Gel                 Composition                                                                           lbs/bbl                                                                            Mud    (°F.)                                                                          (°F.)                                                                       (psi)                                                                              (ml)   Cake    (cps)                                                                              (lbs/100                   __________________________________________________________________________                                                         ft.sup.2)                  AA-AM  5   2000 ppm Ca                                                                           --     300 500 225    26/16" thick                                                                           23  17 9   29                  AA-ITA 5   2000 ppm Ca                                                                           --     300 500 24     2/16" thick                                                                            36  15 4   11                  AA-AM  5   2000 ppm Ca                                                                           400    300 500 79     1 12/16" thick                                                                         34  11 3   12                  AA-ITA 5   2000 ppm Ca                                                                           400    300 500 28     3/16" thick                                                                            19   5 3    4                  AA-AM  6   2000 ppm Ca                                                                           --     300 500 80     1 8/16" thick                                                                          41  32 11  33                  AA-ITA 6   2000 ppm Ca                                                                           --     300 500 22     1/16" thick                                                                            54  26 10  44                  AA-AM  6   2000 ppm Ca                                                                           400    300 500 102    2" thick                                                                               39  50 70  128                 AA-ITA 6   2000 ppm Ca                                                                           400    300 500 20     5/16" thick                                                                            38  44 43  68                  Seawater Mud II                                                                AA-AM  5          --     300 500 110    1" thick                                                                               28   9 3    9                  AA-ITA 5          --     300 500 42     7/16" thick                                                                            10   1 1    1                  AA-AM  5          400    300 500 67     11/16th thick                                                                          10   1 1    2                  AA-ITA 5          400    300 500 31     6/16" thick                                                                             9  11 9   16                  AA-AM  6          --     300 500 56     13/16" thick                                                                           30  12 3    5                  AA-ITA 6          --     300 500 33     5/16" thick                                                                            34  60 20  42                  AA-AM  6          400    300 500 37     4/16" thick                                                                            11   3 1    4                  AA-ITA 6          400    300 500 32     6/16" thick                                                                             9  11 8   13                  __________________________________________________________________________

While the invention has been described with reference to certain specific embodiments thereof, it is understood that it is not to be so limited since alterations and changes may be made therein which are within the full intended scope of the appended claims. 

What is claimed is:
 1. A copolymer of (1) from about 80% to about 98% by weight of acrylic acid and (2) from about 2% to about 20% by weight of itaconic acid, said copolymer having a weight average molecular weight of between about 100,000 to about 1,000,000, being in the form of its free acid or partially or completely sodium, potassium or ammonium neutralized salt and being at least water dispersible, said partially neutralized salt being prepared by reacting carboxylic groups present in said acrylic acid and said itaconic acid with neutralizing agent, said copolymer being useful as a fluid loss control additive for aqueous drilling fluids.
 2. The copolymer of claim 1 wherein said weight average molecular weight is between about 100,000 to about 500,000.
 3. The copolymer of claim 2 in which said copolymer is a copolymer of about 95% by weight acrylic acid and about 5% by weight itaconic acid in the form of its free acid or partially or completely sodium, potassium or ammonium neutralized salt.
 4. The copolymer of claim 2 in which said copolymer is a copolymer of about 90% by weight acrylic acid and about 10% by weight itaconic acid in the form of its free acid or partially or completely sodium, potassium or ammonium neutralized salt.
 5. The copolymer of claim 2 in which said copolymer is a copolymer of about 80% by weight acrylic acid and about 20% by weight itaconic acid in the form of its free acid or partially or completely sodium, potassium or ammonium neutralized salt.
 6. The copolymer of claim 2 in which said copolymer is a copolymer of about 98% by weight acrylic acid and about 2% by weight itaconic acid in the form of its free acid or partially or completely sodium, potassium or ammonium neutralized salt. 